Field of the Invention
The present invention relates to nuclear magnetic resonance (NMR) analysis and more specifically to enclosures for holding subterranean core samples suitable for NMR analysis of the subterranean core samples.
Description of Related Art
NMR analysis of rock core samples extracted from subterranean wells (hereinafter referred to as “rock core samples” or “core samples”) is routinely performed in laboratories in order to examine the T2 relaxation time distribution and other properties of the rock and bound fluids in the respective core samples. These measured properties can be interpreted to give a wide range of information on the pore size distributions and geometry, effective and free-fluid porosities, fluid saturations and distributions, capillary-bound water, permeability, oil viscosity, free fluids, mobile oil, gas and water, clay bound water, and producible fluids and fluid types. Those familiar with the art recognize the significance of these measurements.
NMR analysis of a core sample can be performed while injecting fluids (such as water or surfactant solutions) into the core sample with the use of an appropriate NMR compatible core holder with flow-through capability. This configuration allows for monitoring the recovery of oil from the core sample due to such injection. Whilst such analysis can be performed at realistic reservoir conditions (temperature and pressure), it is difficult to attain in situ analysis of live oil (i.e., oil containing volatile hydrocarbons, gaseous hydrocarbons (e.g. methane) dissolved in solution, and other gases such as H2S and CO2 in solution) because the system will out-gas at reduced pressures (relative to the reservoir). This can prevent a direct comparison between laboratory-scale measurements and data obtained from NMR well logging tools.
Additionally, core samples are often cleaned via processes such as Soxhlet extraction, which alters the wettability of the core sample. It is therefore necessary to restore the natural wettability state prior to the NMR analysis, and this is achieved typically by saturating the core sample with dead oil and then ageing it at an elevated temperature and pressure for several weeks.
If a core sample is to be tested with live oil at reservoir conditions, the core sample must be saturated at elevated pressure to preserve the dissolved gases. It is then necessary to maintain this pressure during the NMR experiments. In the current state of the art, the core sample is loaded into a core holder. An exemplary flow-thru Hassler-type biaxial core holder 11 is shown in FIG. 1. The Hassler-type biaxial core holder is a core holder that applies common radial and axial pressure to the core sample. The core holder 11 includes a tubular vessel 13 that supports an internal flexible sleeve 15 that floats inside the tubular vessel 13 with an annular gap 17 therebetween. Both the tubular vessel 13 and the flexible sleeve 15 are constructed from a material that is non-magnetic and substantially transparent to the radio frequency (RF) radiation employed for NMR analysis. For example, the tubular vessel 13 can be constructed from a glass fiber-reinforced plastic using an epoxy resin or PEEK (polyetheretherketone) as the matrix. The flexible sleeve 15 can be constructed from perfluorinated rubber such as Chemraz®, available from Greene, Tweed and Company of Kulpsville, Pa., USA, or Kalrez®, available from E. I. du Pont de Nemours and Company of Wilmington, Del., USA, or partially-fluorinated rubber such as Viton®, also available from E. I. du Pont de Nemours and Company. The interior space of the flexible sleeve 15 receives the core sample 19. The annular gap 17 defines a radial pressure chamber for applying radial pressure to the core sample 19 via the annular wall of the flexible sleeve 15. The opposed ends of the flexible sleeve 15 are sealed by corresponding end platens 21A, 21B. End caps 23A, 23B are secured to opposed ends of the tubular vessel 13. An interior space 25 between the end cap 23A and the end platen 21A on one end of the flexible sleeve 15 defines an axial pressure chamber for applying axial pressure to the core sample 19 via the end platen 21A. One or more pressure ports (for example, two shown as 27A, 27B) are provided on the annular wall of the tubular vessel 13. The pressure port(s) are fluidly coupled to both the radial pressure chamber 17 and the axial pressure chamber 25 by fluid passageways 29A, 29B. The pressure port(s) 27A, 27B receive pressurized fluid that is substantially transparent to the RF radiation employed for NMR analysis (such as the any of the Flourinert® family of per-fluorinated hydrocarbons available from 3M Company of Minneapolis, Minn., USA). The pressurized fluid fills the radial pressure chamber 17 and the axial pressure chamber 25 to apply both radial and axial pressure to the core sample 19. The radial pressure prevents out-gassing and fluid flow around the core sample 19 during the acquisition of NMR measurements of the core sample 19. The temperature of the core sample 19 is controlled by the temperature of the pressurized fluid supplied to the pressure port(s). Pressure and temperature is typically regulated by a reservoir of pressurized fluid that is fluidly coupled to the pressure port(s). One end cap 23A includes a fluid inlet port 31A that communicates with a fluid inlet path 33A extending through the adjacent end platen 21A and into the interior space of the flexible sleeve 15 for the supply of fluid (such as oil) to the core sample 19. The opposite end cap 23B includes a fluid outlet port 31B that communicates with a fluid outlet path 33B extending through the adjacent end platen 21B and into the interior space of the flexible sleeve 15 for the discharge of fluid from the core sample 19. The fluid inlet port 31A and the fluid outlet port 31B allow the core sample 19 to be prepared for NMR analysis. Such preparation can involve saturating the core sample 19 with oil at the required conditions for live oil studies. Such prior art flow-thru Hassler-type biaxial core holders are sold commercially by ErgoTech Limited of Conwy, United Kingdom.
The core holder can be confined by a load frame and is surrounded by a permanent magnet (four iron yoke pillars supporting two opposed magnetic poles) while the core is prepared. The permanent magnet is used for NMR analysis and remains in position surrounding the core holder during the preparation of the core sample and during the NMR analysis.